Speedometer jumping around help

Hey I have a 73 duster I’m working on with my son. Have been asking a lot of questions lately as this car needs a lot of work. It has a 360la with a 904 automatic tranny. My question is the speedometer jumps all around gets a bit more stable at higher speed but not by much. It got worse after I picked it up from transmission place for a rebuild, but it was bad before I took it to them too. Can someone point me in the right direction. Do I need to find a new cable or is it something where it attaches to the transmission??anyone have experience with something similar? Thanks!